Structural materials such as concrete and mortar have high resistance to compression forces, but low resistance to bending and shear forces. To compensate for these weaknesses, structures made from these materials also include reinforcing bars to reinforce the structure against bending and shear forces which may cause the structure to fail. This is especially true of concrete and mortar structures which are used in highway and building construction. The reinforcing member is usually an iron or steel rod or bar which is imbedded in the structure typically by pouring the wet concrete or mortar into a mold containing the bar, typically as part of a network of reinforcing bars.
Over time, the reinforcing bars tend to lose their tensile strength due to oxidation. Oxidation may be hastened by contact with salts, such as may occur in coastal areas, or where salt is applied to road surfaces to remove ice. Eventually, the reinforcing bars may be corroded to the point where they can no longer provide effective stress protection. Structural failures due to such corrosion, such as in some highway bridges, is not uncommon.
Attempts to counteract the corrosion of structural reinforcing members in cement structures without completely rebuilding the structure have been proposed. In one approach, the corroded reinforcing bars in a structure are treated by applying an electric current to the bars. The reinforcing bars serve as a cathode, and a dimensionally-stable electrode or a conductive polymer electrode on the structure surrounding the member serves as an anode. The application of an electric current reverses the electrochemical oxidation of the reinforcing members in the structure. However, the method does not provide long-term protection.
U.S. Pat. No. 4,092,109 discloses a method for inhibiting corrosion in reinforcing members by including applying calcium nitrite in mortar and concrete mixtures used in reinforced structures. This method is not applicable, of course, to preexisting structures. The patent also discloses a method for inhibiting corrosion in a reinforced cement structures, by soaking the structure with a 25% solution of calcium nitrite in water. This method is limited by the slow the rate of migration of NO.sub.2.sup.- ions into the cement matrix.